The present invention relates to electric motors and in particular to rotatable permanent magnets in a rotor to reconfigure the motor from an asynchronous induction motor at startup into a synchronous motor for efficient operation.
A preferred form of electric motors are brushless AC induction motors. The rotors of induction motors include a cage (or squirrel cage resembling a “hamster wheel”) rotating inside the stator. The cage comprises axially running bars angularly spaced apart on the outer perimeter of the rotor. An AC current provided to the stator introduces a rotating stator magnetic field in the stator, and the rotating field inductively induces current in the bars. The current induced in the bars then cooperate with the same stator magnetic field to produce torque and thus rotation of the motor.
The introduction of current into the bars requires that the bars are not moving (or rotating) synchronously with the rotating stator magnetic field because electromagnetic induction requires relative motion between a magnetic field and a conductor in the field. As a result the rotor must slip with respect to the rotating stator magnetic field to produce torque and the induction motors are thus asynchronous motors.
Unfortunately, low power induction motors are not highly efficient, and lose efficiency under reduced loads because the amount of power consumed by the stator remains constant at low loads.
One approach to improving induction motor efficiency has been to add permanent magnets to the rotor. The motor initially starts in the same manner as a typical induction motor, but as the motor reached its operating speed, the stator magnetic field cooperates with the permanent magnets to enter synchronous operation. Unfortunately, the permanent magnets are limited in size because if the permanent magnets are too large, they prevent the motor from starting. Such size limitation limits the benefit obtained from the addition of the permanent magnets.